How Ears Tune In and Out Certain Sounds

Ever wonder how humans with normal hearing are able to "hear" what one person is saying in a crowded room where many people are talking? How is it that the ear is able to tune in and out of conversations going around, especially when everybody in a room is talking equally loud?

To understand this ear "tuning" ability, one needs to first understand that the ear incorporates two "aspects" that allow for good hearing: a volume knob and an antenna. Should the volume knob not work, hearing aids boost the frequencies where hearing loss is present. However, if the antenna does not work, simply increasing the volume does not necessarily make sounds/speech more understandable. This situation is analogous to an old radio where turning the volume up on a station with a lot of static does not make sounds more understandable (fiddling with the antenna to get a clear signal is better).

We have a very good understanding the volume knob of the human ear. What we do NOT have a good understanding (nor how to fix) is the antenna.

However, researchers have begun to clarify how the human ear "fiddles" the antenna to make certain sounds more understandable relative to others providing humans the ability to tune into what one person is saying in a loud restaurant.

Apparently, part of the answer is the tectorial membrane found within the cochlea (membrana tectoria in the Gray's illustration below).

Note that this membrane found inside the cochlea rests on the inner and outer hair cells which allow for hearing (the volume knob). When the ear hears a sound, "waves" are created within the cochlea stimulating the hair cells which allow us to hear. Depending on which hair cells are stimulated, a specific sound frequency is heard.

Using mutant mice, MIT researchers have determined that the viscosity of tectorial membrane depends on the size and distribution of tiny pores. This viscosity provides mechanical filtering that helps to sort out specific sounds by influencing the speed and decay of tectorial membrane waves when a sound is heard.

Although the precise way how the tectorial membrane allows for such exquisite frequency selectivity is still not fully understood, this may be the first step towards that comprehension.

Because once we firmly understand how the ear's antenna works, we hopefully can make better hearing aids that not only act as a volume knob, but can also fiddle the antenna as well.

Reference:
Porosity Controls Spread of Excitation in Tectorial Membrane Traveling Waves. Biophysical Journal, Volume 106, Issue 6, 1406-1413, 18 March 2014 doi:10.1016/j.bpj.2014.02.012